A lithium ion cell, as a secondary cell, has been utilized as a power source of various portable devices such as a mobile phone, however, in recent years, research and development of large size batteries assuming automotive applications and the like has become active. Therefore, it has become indispensable to further increase energy density of an existing lithium ion cell. Accordingly, use of a high capacity silicon material has attracted the attention, as an active material, instead of a carbon-based material. High capacity of silicon is considered to be due to providing higher electric capacity, as compared with the case of using carbon, because silicon can cause an alloying reaction with lithium electrochemically at room temperature.
However, silicon has been known to cause large volume change (enlarge to 3 times or more) on charge-discharge, when used as an active material. And, this volume change generates destruction of an electrode structure in charge-discharge, and leads to destruction of an electrode. As a result, it had a problem of shortening of cycle characteristics (lifetime) or the like. In addition, it had also a problem of larger self-discharge or the like, as compared with a graphite electrode.
On the other hand, various attempts of using a binder have been made, aiming at increasing capacity or enhancing stability performance of a cell (PATENT LITERATURE 1 and 2). However, these targeted active materials were mainly carbon materials, and does not aim at resolving the above-described problems in the case of using silicon.